This invention relates to fiber optical sensors and, more particularly, to fiber Bragg grating sensors used to measure temperature and strain changes. An area that has been investigated closely is the measurement of longitudinal strain and temperature. M. Xu, H. Gieger and J. P. Dakin in xe2x80x9cMultiplexed Point and Stepwise Continuous Fiber Grating Based Sensors: Practical Sensor for Structural Monitoring?xe2x80x9d, Proceedings of SPIE, Vol. 2294, p. 94, 1994 described the usage of dual overlaid fiber gratings to measure strain and temperature. The major difficulty with this approach is that to obtain reasonable accuracy the wavelengths must be widely separated. Fiber etalon based strain sensors based on air cavities that are sold commercially by FISO and Luna offer relative immunity to temperature sensitivity but are difficult to multiplex into a single fiber line.
Fiber grating systems are needed that can measure either temperature or strain independently that can be easily multiplexed along a single fiber line without cross sensitivity.
In the present invention fiber grating strain and temperature sensors are described that are designed to measure only strain or only temperature eliminating the need for compensation. This is accomplished by using a fiber optic sensor, which includes an interference energy analyzer and a series of double Bragg grating elements that are used to measure strain and temperature distribution along an optical fiber. Each double-Bragg grating element consists of two Bragg gratings, separated by a distance unique to each element. The interference energy analyzer calculates the energies of the interference patterns, which are created by beams reflected from double-Bragg grating elements. The energy of the interference signal changes when the gratings in one element non-uniformly change its parameters due to non-equal temperature or strain influence on two gratings. This is accomplished by formed matched pairs of fiber gratings that act as etalons and isolating one of the fiber gratings from strain or temperature. By writing gratings into birefringent fiber and forming matched pairs it is also possible to monitor transverse strain and offset temperature effects. By converting the interaction of moisture or chemical species into strain on the fiber gratings these parameters may also be measured while compensating for temperature effects on the fiber gratings.
Therefore, it is an object of the invention measure strain and temperature using fiber grating sensors with reduced cross sensitivity.
Another object of the invention is to allow transverse strain measurement with reduced temperature sensitivity.
Another object of the invention is to provide a means to measure moisture or chemical content of the environment with reduced temperature sensitivity.